Seismic Vulnerability Assessment of Highway Systems

This article presents research conducted on the development of a seismic
vulnerability assessment procedure for highway systems funded under NCEERs Highway
Project. It represents work in progress and the data and models presented for the Memphis
highway system are preliminary. They are for illustration purposes only and are subject to
change. A more complete version of this paper will appear in the proceedings of the
"National Seismic Conference on Bridges and Highways," sponsored by the Federal
Highway Administration and Caltrans, to be held on December 10-13, 1995 in San Diego,
California. Comments and questions should be directed to Stuart Werner, Dames and Moore,
at (415) 896-5858.

Framework of Seismic Vulnerability Assessment Procedure

General Description
The general Seismic Vulnerability Assessment (SVA) procedure for a highway system is shown
in figure 1. The procedure involves four main steps, which are: (1) initialization of the
SVA; (2) development of system SVA results for each scenario earthquake and simulation
specified under Step 1; (3) incrementation of the simulations and the scenario earthquakes
and repeat of Step 2; and (4) aggregation of the SVA results for all earthquakes and
simulations. Key to this process is a GIS database, which comprises several modules that
contain the data and models for implementing the various steps of the system SVA.

This SVA procedure has several desirable features. First, it would be carried out
within a GIS framework, which will enhance data management, analysis efficiency, and
display of analysis results. Second, the GIS data base would be modular, in order to
facilitate the incorporation of improved data, procedures, and models, as they are
developed from future research and development efforts. Third, the procedure would be able
to consider the effects of uncertainties in the earthquake characterization, hazard
models, and vulnerability models, and would have the capability of developing aggregate
SVA results that could be either deterministic or probabilistic, depending on user needs.
This range of results would facilitate the usefulness of the SVA for seismic retrofit
planning, prioritizing, and criteria development for an existing highway system.

With this as background, the remainder of this section outlines the basic features of
the GIS data base for the SVA procedure, together with each of the above four steps of the
procedure that are listed above.

Assessing the impacts of each scenario earthquake on traffic flows throughout the
system.

The various modules that comprise the data base are summarized in the paragraphs that
follow.

System Module. The system module would contain the basic data needed to define
the system for the subsequent steps of the SVA. These data would describe: (a) system
network configuration and linkages; (b) roadway widths (number of lanes each way); (c)
traffic flows, capacities, and volumes for the roadways within the system; (d) component
types and locations; (e) origin-destination zones; and (f) any special characteristics of
the system, such as roadways designated as critical for national defense or for emergency
response.

Hazards Module. The hazards module would contain the data and models needed to
evaluate the seismic and geologic hazards throughout the system. The geologic data might
include: (a) locations, earthquake activity rates, earthquake magnitude potentials, and
tectonic displacement data for major faults and/or seismic zones in the region; (b)
locations and topographic information for hills or valleys near the system that could be
prone to landslide; and (c) data describing regional geology and local soil conditions
throughout the system that would be needed to estimate ground shaking and potential ground
movement due to liquefaction, landslide, etc. In addition, this module would include: (d)
ground motion attenuation relationships; (e) models for estimating local soil effects on
ground shaking; (f) landslide, liquefaction, and fault rupture models; and (g) models for
characterizing uncertainties in ground shaking and ground movement.

Component Module. The component module would contain the data and models needed
to estimate component vulnerabilities and their uncertainties. In this, component
vulnerabilities would be characterized using loss models and functionality models. Loss
models would estimate direct losses (repair and replacement costs) due to earthquake
damage to the components. Functionality models, which are needed for post-earthquake
traffic flow analysis, would represent the number of lanes open to carry traffic along
each roadway link in the system at various times after an earthquake, together with any
reduced speed limits due to earthquake damage. Input data for developing these models
would also be contained in this module. These data would include: (a) structural attribute
data needed to evaluate the seismic performance of each component under various levels of
ground shaking and ground movement; and (b) damage repair strategies, costs, and traffic
impacts, including the number of traffic lanes to be closed during repair, the durations
of these lane closures, and the reduced speed limits for traffic in the repair areas.

Traffic Management Module. The traffic management module would accommodate
information on post-earthquake traffic management procedures for alleviating traffic
congestion after an earthquake. For example, experience following the Northridge
earthquake showed that emergency traffic management procedures implemented by
transportation planners and engineers from the City and County of Los Angeles and from
Caltrans were very effective in reducing traffic congestion from earthquake damage.

Traffic Flow Module. The transportation flow module would contain the traffic
model and system analysis procedure to be used for estimating earthquake effects on
traffic flows throughout the system, for a given post-earthquake system state. This would
consist of a system traffic forecasting methodology that would estimate effects of each
scenario earthquake and simulation on such quantities as travel times, travel distances,
and travel paths. These quantities could be estimated on an overall system basis (which
would serve as a rough indicator of overall system performance) and also between selected
origin-destination zones in the system.

Step 1: Initialization of Analysis
With the development of the GIS data base for the system to be analyzed, the actual SVA
itself could be initiated. This would incorporate two parts. The first part would involve
the use of earthquake source models (including randomization models) contained in the
hazards module of the GIS data base to define a suite of scenario earthquakes that could
affect the highway system to be analyzed. Uncertainties modeled would include geographic
location of the earthquake source. Other uncertainties that could in principle be
incorporated at this stage could address: (a) magnitude range for each source; (b)
magnitude vs. fault rupture relationship; (c) orientation of rupture source; (d)
directivity of rupture propagation; and (e) earthquake model uncertainties (e.g.,
uncertainties in "a" and "b" values in the Gutenberg-Richter
relationship, or in characteristic earthquake models, time-dependent or time-independent
models, etc.).

The second part of Step 1 would identify an adequate number of simulations for each
scenario earthquake. In this, a "simulation" is defined as a complete set of
hazards and component input parameters, in which the values of the parameters have been
changed in accordance with the uncertainty characterizations contained in the hazards and
component modules. For each simulation, a separate system SVA would be carried out under
Step 2 (as described below). This process would be repeated until a sufficient number of
simulations have been considered for each parameter to permit an evaluation under Step 3
of how the system SVA results are impacted by uncertainties in the parameter values.
Effects of the uncertainties in each parameter can in principle be treated in this way. In
what follows, each simulation for earthquake m (m=1,2,....M) is designated as n(m), (where
n(m) = l(m), 2(m)....N(m), and N(m) is the total number of simulations for earthquake m).

Step 2: System Analysis for Earthquake m and Simulation n(m)
The next step in the SVA procedure would consist of a system analysis for each simulation
associated with each scenario earthquake. For each simulation, the analysis would involve
the following evaluations: Hazard Evaluation. First, the data and models contained in the
hazards module of the GIS data base would be used to estimate the earthquake ground
motions and geologic hazards throughout the system.

Direct Loss and System State Evaluation. Once the hazards are estimated, the
component module in the GIS data base would be used to evaluate the direct losses and the
system state associated with the mth earthquake and the n(m)th
simulation for that earthquake. The direct losses would indicate the total cost for repair
or replacement of damaged components within the system. The system state (defined at
various times after the earthquake) would indicate the number of lanes that remain open to
traffic along each roadway in the system, and any reduced speed limits within the system
while the damage is being repaired.

Traffic Flow Evaluation. The system traffic flow models and traffic forecasting
methodology contained in the traffic flow module would be applied for each system state,
in order to assess how travel times, travel distances, and travel paths throughout the
system and between its origin-destination zones would be impacted by the earthquake damage
associated with the given system state. In principle, a local or regional socioeconomic
model could be added at this stage, to evaluate broader social and economic impacts of the
earthquake damage.

Step 3: lncrementation of Simulations and Scenario Earthquakes
This step simply represents the process wherein the system analysis from Step 2 is
repeated for each simulation associated with each scenario earthquake.

Step 4: Aggregate System Analysis Results
This final step in the SVA process would be carried out after the system analyses for each
simulation and each scenario earthquake have been completed. In this step, the results
from all simulations would be aggregated and displayed. Depending on user needs, these
aggregations could focus on the seismic risks associated with the total system or with
individual components. Furthermore, the system or component results could be provided for
individual simulations and/or for the broader (probabilistic) range of simulations. For
research purposes, the impacts of incorporating variabilities into the SVA will be of
considerable interest. For other purposes, such as the planning of seismic strengthening
programs for existing highway systems, outputs can be adapted and/or simplified in
accordance with the particular requirements of each user audience.

Demonstration Seismic Vulnerability Assessment
The above SVA procedure has been applied to a Memphis area highway system (fig. 2) in
conjunction with currently available data and models, to demonstrate the application of
the procedure and the type of results that can be obtained, and to also provide a basis
for identifying and prioritizing research needs to be addressed in subsequent years of the
NCEER Highway Project. It is noted that, because of the preliminary nature of much of the
currently available data and models, the results of this demonstration SVA not be
interpreted as a prediction of the seismic performance of this Memphis area highway system
at this time. As improvements to these data and models are developed under the Highway
Project, the reliability of the system seismic performance estimates should increase
substantially.

System
The city of Memphis is located in the southwestern corner of Tennessee, just east of the
Mississippi River and just north of the Tennessee-Mississippi border. Because of its
proximity to the New Madrid seismic zone, the potential seismic risks to the Memphis area
are well recognized and have been studied extensively (e.g., A & H, 1982; Desmond,
1994). The Memphis area highway system evaluated under this demonstration SVA (fig. 2a)
includes the beltway of interstate highways that surrounds the city, the two crossings of
the Mississippi River (at Interstate Highways 40 and 55), major roadways within the
beltway, and highways just outside of the beltway that extend to important transportation,
residential and commercial centers to the south, east, and north. Locations of such
centers within this system are shown in the Origin-Destination (O-D) zone map provided in
figure 2b. The system contains a total of 286 bridges.

Assumptions
As noted earlier, this demonstration SVA is based on currently available data and models
only. Because the data and models are very preliminary at this time, it has been necessary
to incorporate certain simplifying assumptions into this SVA. These assumptions are
summarized below.

Scenario Earthquakes. This demonstration SVA was carried out for four scenario
earthquake events only. These four earthquakes represent a range of different moment
magnitude levels and locations in the region surrounding Memphis (fig. 3a), and are as
follows: (a) Earthquake A-which has a moment magnitude MW = 7.5 (corresponding
to a repeat of the largest earthquake in the 1811- 1812 sequence) and is located at the
southern end of the New Madrid seismic zone (Zone A in fig. 3a); (b) Earthquake B -which
has a moment magnitude MW = 6.5 and is located near the center of Zone A; (c)
Earthquake C - which has a moment magnitude MW = 6.0 and is located in Zone B
to the west of Zone A; and (d) Earthquake D - which has a moment magnitude MW =
5.5 event and is located in Zone B east of Zone A. The distances from the assumed
epicenters of these various earth quakes to the closest and furthest points within the
Memphis highway system range from about 35-50 km (for Earthquake D) to about 110-125 km
(for Earthquake C). These earthquakes are described further in Werner and Taylor (1995).
This article provides results for Earthquake D only.

System Module. The only system components that have been considered in this
demonstration SVA are bridges and roadways. The system does not contain any tunnels, and
other system components (e.g., retaining walls, etc.) have not been considered. The
configuration and layout of the highways within the system were obtained as part of a GIS
database provided by the University of Memphis. An extensive data base of structural
attributes relevant to seismic performance have been compiled for most of the bridges in
the area, and are provided in Werner and Taylor (1995). The traffic flow and volume data,
roadway traffic capacities, and O-D zones within the system were provided by the Memphis
and Shelby County Office of Planning and Development (OPD). The traffic flow data were
from their 1988 traffic forecasting model.

Hazards Module. The only seismic and geologic hazard that has been considered in
this SVA is ground shaking. Potential hazards from liquefaction, landslide, and associated
ground movement have not been included because of a lack of suitable data for carrying out
such evaluations over a spatially dispersed region and for a range of scenario earthquake
events. The ground shaking hazard was represented in terms of peak ground acceleration
(PGA). It was estimated in two steps. First, bedrock accelerations at each bridge site due
to each scenario earthquake were estimated using the attenuation equation developed by
Hwang and Huo (1994). Then, effects of local soil conditions at each bridge site were
represented by multiplying the bedrock accelerations by local geology factors developed by
Martin and Dobry (1994) for various site categories and bedrock acceleration levels. This
was based on the local geology mapping of the area carried out at the University of
Memphis (fig. 3b), and is contained in the GIS data base for this demonstration SVA (Hwang
and Lin, 1993; Tarr and Hwang, 1993). Figure 4 shows the resulting bedrock and ground
surface peak accelerations for Earthquake D.

Component Module - Loss Model. In this demonstration SVA, loss models previously
developed under the ATC-25 project for conventional highway bridges were used to estimate
direct losses for each bridge in the system due to each earthquake (ATC, 199 1). In these
models, the direct losses depend only on whether the bridge has simple spans or is
continuous/monolithic; i.e., other bridge structural attributes that could impact seismic
performance have not been considered.

Component Module - Functionality Model. Because of a lack of available and
suitably compiled data pertaining to post-earthquake traffic flows, repair procedures, and
repair times for a given type and degree of bridge damage, only a very simple
functionality model could be used for this demonstration SVA. Accordingly, the
functionality model that was used represents the number of lanes open at discrete times
after an earthquake, as a function of PGA and the original number of lanes along the
bridge. Two different models were developed in accordance with the ATC-25 conventional
highway bridge designations-one for bridges with simple spans and one for
continuous/monolithic bridges. Reductions in traffic speeds were not considered at this
time. In addition, to illustrate that system performance can vary with time after the
earthquake, functionality models were developed for two discrete times. The first was
intended to represent a time shortly after the earthquake, before any repairs have been
made but after undamaged bridges had been reopened and lane closures to accommodate
immediate post-earthquake repair had been established. This time has been arbitrarily
assumed to be three days after the earthquake (recognizing that this may be optimistic).
The second time was assumed to represent a more extended time after the earthquake, when
some bridge repair has been made and at least some lanes of the damaged bridges have been
reopened to traffic. This time was arbitrarily assumed to be six months. These approximate
functionality models do not represent all of the possible causes of bridge and roadway
closure after an earthquake, nor do they consider alternative bridge repair strategies
that may be employed (together with their associated costs, durations, and impacts on
traffic flow). The functionality models that were used are discussed in detail in Werner
and Taylor (1995).

Traffic Management Module. In view of the absence of information addressing
emergency traffic management procedures that would be implemented in the Memphis area
after a major earthquake, the traffic management module of the GTS data base was not
included in this demonstration SVA.

Traffic Flow Module. Our analysis of the impacts of each scenario earthquake on
traffic flows within the Memphis area highway system was carried out using the MINUTP
traffic forecasting software (Comsis, 1994). This software was selected because it is the
procedure used at the Memphis-Shelby County Office of Planning and Development (OPD), and
all traffic data for the region was available in the input format for this software.
Although this procedure appeared to provide reasonable results for the various cases that
were run (Werner and Taylor, 1995), it has the significant disadvantage of not being
compatible with our GIS data base. Because this greatly increased the efforts required to
develop suitable input data for the traffic flow analyses and to interpret the analysis
results, the identification (or development, if necessary) of a suitable GIS-compatible
traffic flow methodology is recommended as a high priority SVA research area.

Results: Direct Loss Estimates, Scenario Earthquake D
In accordance with the ATC-25 model used in this demonstration SVA, direct losses due to
damage to the systems bridges are represented as a damage ratio, DMG (%), which is
defined as the ratio of the repair cost for each bridge to its total replacement cost.

The damage ratios for each of the 286 bridges in the Memphis area highway system due to
each scenario earthquake are tabulated in Werner and Taylor (1995). To roughly compare the
relative effects of each earthquake on the direct losses throughout the system, average
damage ratios were computed (averaged over all of the 286 bridges) for each earthquake.
This article provides results for Earthquake D only, for which this average damage ratio
was 37.4%. From the results provided in Werner and Taylor (1995), this damage ratio turned
out to be much larger than that computed for Earthquake C (whose effects on the system
were relatively minor), and was slightly larger than the average damage ratio computed for
Earthquake B. Only Earthquake A, which was by far the most severe of the four scenario
earthquakes considered, resulted in damage ratios that were larger (by a substantial
amount) than those due to Earthquake D.

Results: Traffic Flows,
Scenario Earthquake DOverview of Seismic Vulnerability Assessment Procedure. This seismic
vulnerability assessment estimated how earthquake damage to the Memphis area highway
system due to each scenario earthquake impacted traffic flows in the area. The analysis
consisted of two parts. First, the PGAs estimated for each scenario earthquake were
applied to the functionality models, in order to estimate the state of the system at times
of three days and six months after each earthquake (in terms of the number of available
lanes along each roadway in the system). Then, the effects of any reductions in the
available lanes (due to earthquake damage) on traffic flows throughout the system were
estimated by using the MINUTP transportation forecasting software, together with a
regional traffic capacity and flow data base developed at the Memphis and Shelby County
OPD. From this, travel times and distances throughout the system after each earthquake
were compared to pre-earthquake travel times and distances (in which all travel times and
distances are average values for a 24 hour period). Two sets of comparisons were made. One
corresponded to an overall travel time and distance for the entire system, which are
computed as the sum of the travel times and distances respectively between all
origin-destination (O-D) zones in the system (fig. 2b). This set of comparisons provides
an approximate measure of the impacts of each earthquake on overall system performance.
The second set of comparisons involved a breakdown of these total travel times and
distances for particular key O-D zones highlighted in figure 2b. These latter comparisons
indicate the spatial distribution of the earthquake impacts throughout the system, and
also show how travel to and from these important O-D zones are impacted by earthquake
damage to the highway system.

System State Results. Based on the PGA estimates obtained throughout the system
due to each scenario earthquake, together with the preliminary functionality models, the
system state after each earthquake was estimated. This system state is defined as the
number of lanes open along each link in the system. The pre-earthquake system state and
example system state results for times of three days and six months after Earthquake D are
shown in figures 5 and 6. These figures show that, although Earthquake D is the smallest
of the four scenario earthquakes (MW = 5.5), the gross models used for this SVA
estimate that the proximity of this earthquake to the northern segment of the Memphis area
highway system results in extensive roadway and lane closures in this segment, with lesser
impacts on other sections of the system.

Overall System Travel Times. Table 1 shows that, as a result of the estimated
bridge damage due to Earthquake D, overall system travel times three days after the
earthquake are nearly 34 percent larger than the pre-earthquake values. Six months after
the earthquake, the bridge repairs within that time have reduced the overall system travel
time; however it is still nearly 20 percent larger than the pre-earthquake value.

Table 1: Effects of Earthquake D on Total System Travel Times and
Distances

Parameter

Pre-Earthquake Value

T = 3 days

T = 6 months

Value

Percent Increase over Pre-EQ

Value

Percent Increase over Pre-EQ

Total vehicle hours traveled over 24-hour period (incl.
congestion)

3.73 x 105

4.99 x 105

33.8

4.46 x 105

19.6

Total travel distance (mi) over 24-hour period

15.5 x 106

15.6 x 106

Small

15.6 x 106

small

Note: T = Time after
earthquake at which system-wide impacts are estimated.

Overall System Travel Distances. Table 1 shows that over-all
system travel distances are not sensitive to the estimated bridge damage due to Earthquake
D, despite the fact that the total number of trips estimated over a 24-hour period by
MINUTP (solely on the basis of demographics) was nearly the same for the pre-earthquake
system and for each scenario earthquake. This lack of change of travel distances, despite
significant increases in travel times, is no doubt due to the types of more direct but
less-time efficient routes that would need to be taken after an earthquake. For example,
if faster but less direct routes along interstate highways and beltways that would
ordinarily be used are closed because of bridge damage, slower but more direct routes
along city streets with no damaged bridges would instead need to be used.

O-D Zone Travel Times. Table 2 shows that three days after Earthquake D, the
travel times between the zones listed in the table are estimated to be, on the average,
nearly 16 percent larger than those for the pre-earthquake system. The travel time
increases due to damage from this earthquake are estimated to be largest for northernmost
of the highlighted zones, which are at Shelby Farms (Zones 249 and 252), Bartlett (Zone
264), and the Covington Pike (Zone 274). Six months after Earthquake D, table 2 shows that
the travel times to and from these zones have been reduced substantially, and are now only
5.3 percent larger than the pre-earthquake values.

O-D Zone Travel Distances. As for the overall system travel distances, the
travel distances to and from the highlighted O-D zones are not sensitive to damage from
Earthquake D (Werner and Taylor, 1995).

Table 2: Effects of Earthquake D on Travel Time to or from
Designated Origin-Destination Zones (Over 24 Hour Time Period)

ORIGIN-DESTINATION ZONE

No.

PRE-EQ TRAVEL TIME
(HOURS)

3 DAYS AFTER EARTHQUAKE

6 MONTHS AFTER EARTHQUAKE

Description

Travel Time (hrs)

Percent Increase over Pre-EQ Time

Travel Time (hrs)

Percent Increase over Pre-EQ Time

Government Center (downtown Memphis)

7

128

143

11.7

133

3.9

8

122

141

15.6

130

6.6

Medical Center

25

122

136

11.5

127

4.1

26

114

129

13.2

121

6.1

27

114

129

13.2

121

6.1

28

115

129

12.2

121

6.2

29

119

133

11.8

124

4.2

University of Memphis

111

119

131

10.1

122

2.5

President's Island (Port)

151

138

153

10.9

144

4.3

Memphis Airport

188

136

150

10.3

142

4.4

Federal Express

189

130

145

11.5

136

4.6

Mall of Memphis

201

127

145

14.2

133

4.7

Hickory Hall

213

171

185

8.2

177

3.5

Poplar-Ridgeway

230

130

148

13.0

136

4.6

231

130

147

13.1

136

4.6

Germantown

236

141

157

11.3

147

4.3

241

176

187

6.3

181

2.8

Shelby Farms

249

169

176

4.1

174

3.0

252

127

211

66.1

152

19.7

Bartlett

264

148

199

34.5

155

4.7

Covington Pike

274

137

181

32.1

151

10.2

TOTALS

2813

3255

15.7

2963

5.3

Acknowledgments
The authors wish to acknowledge Ian Buckle and Ian Friedland of NCEER and Masanobu
Shinozuka of the University of Southern California for their encouragement and helpful
suggestions, Abdul Razak of the Memphis and Shelby County OPD for providing traffic data
and for his help with the MINUTP traffic flow analyses, and Edward Wasserman and his staff
at the Tennessee Department of Transportation in Nashville, Tennessee for providing
valuable bridge data, drawings, and reports from their files. Finally, the authors are
grateful to the following Dames & Moore personnel for their significant contributions
to this research: C.B. Crouse (seismic hazard analysis), Ahmed Nisar (numerical analysis),
and Jon Walton (GIS applications).

ReferencesAllen & Hoshall, Inc. (A&H), (1982) Central United States Earthquake
Preparedness Project (CUSEPP)-An Assessment of Damage and Casualties for Six Cities in the
Central United States resulting from Earthquakes in the New Madrid Seismic Zone, A &
H, Memphis, TN.

Applied Technology Council (ATC), (1991) ATC-25-Seismic Vulnerability and Impact of
Disruption of Lifelines in the Conterminous United States, ATC, Redwood City, CA.